There have been known many dc power supply units for providing a constant dc voltage by converting an inputted dc voltage, equipped with an over-current protection function to limit the output current to a predetermined level.
An example of such over-current protection function is a depending type protection function in which an output voltage versus output current characteristic for limiting the output current is initiated at a predetermined output current level. Although such depending type over-current protection function is usable, the power control transistor of the converter can be destructed by resistive heat generated if a certain constant current continues to flow through it.
A known fold-back current limiting type over-current protection circuit is capable of overcoming the above-mentioned drawback of the depending type protection function. A feature of the fold-back current limiting type over-current protection circuit is characterized in that when the output voltage begins to decrease the output current also decreases simultaneously and that the level of the output current associated with the lowest output voltage is lower than the level of the current at the beginning of the decrease. The feature will be hereinafter referred to as over-current protection scheme. Thus, the output current is restricted to a sufficiently low level by the fold-back current limiting type over-current protection scheme even if a constant current continues to flow, thereby avoiding destruction of the power transistor (see, for example, Japanese Patent Applications (JPA) Laid Open 2002-304225 and 2000-209850).
Constant voltage DC power units equipped with a fold-back current limiting over-current protection circuit are mostly constructed in the form of series power supply units as disclosed in the first reference (JPA Laid Open 2002-304225). As a consequence, although series power supply units can enjoy merits of a fold-back current limiting over-current protection circuit, they are not suitable for use in cases where high power-efficiency is required, since they inevitably suffer from a large power loss that accompanies the voltage control. A switching power supply unit having a transformer interposed between the input and output terminals of a DC power unit is also proposed in the second reference (JPA Laid Open 2000-209850), which is, however, difficult to be miniaturized because of the transformer included, and hence not suitable for use in LSIs.